Continuous annealing method and apparatus



May 28, 1968 D. G. HATCHARD 3,335,946

CONTINUOUS ANNEALING METHOD AND APPARATUS Filed April 16, 1965 V eamelmlaimeemk Gas -L Header 7 Gas Fired Radiant Tubes 94: F lg. l

Fuel Gas gnd lAir a van y Header f welding Station I I: I: I: I: I: Temperature Sensing 6 Means Strip Tension Control Means Induction Coil H h F 'equency Energizotion surce Control Means I400 I Final Annealing I300; e Temperature of Strip i200 L t H M00 P f' Recrys a ization 3. I000 Temperature .3 900 BOO 700 Tinplate Strip Temperature 5 600 During Normal Transit v, 500 Through Furnace 30o v 20o Fig. 2 I00 Radiant Only WITNESSES lnduction INVENTOR 9M4 3MP Donald G. Hatchord Zones Within the Furnace W 4% may AGENT United States Patent 3,385,946 CONTINUOUS ANNEALING METHOD AND APPARATUS Donald G. Hatchard, Arnold, Md., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., :1 corporation of Pennsylvania Continuation-impart of application Ser. No. 146,526,

Oct. 20, 1961. This application Apr. 16, 1965, Ser.

2 Claims. (c1. 219--10.47

ABSTRACT OF THE DISCLOSURE A continuous annealing method and apparatus therefor, wherein metal strip, such as cold-worked low carbon steel tinplate strip, is consecutively heated continuously by travel first through a multiple-loop radiant furnace operated at a temperature below the recrystallization temperature of the metal strip, below 1100 F. in the case of such tinplate strip, and then through an induction heating coil means to provide the upper strip temperature, above 1100 F. and less than 1400 F. in the case of such tinplate strip, under regulation of the heating coil means which prevents exceeding such upper strip temperature during cessation of such travel.

This is a continuation-in-part of application Ser. No. 146,526, filed Oct. 20, 1961, now abandoned.

Present practice in the continuous annealing of lowcarbon steel strip cold-reduced more than to .004 to .015 inch in thickness, for example, involves the passing of the strip through a high thermal inertia heating means such as a thick-walled refractory-lined furnace having radiant heating elements in the form of gas-fired radiant tubes or electric resistance elements, disposed in vertical arrays between upper and lower pass rolls which direct the strip in multiple loops through the furnace for subjection to the radiant heating effect of such arrays for the period of time necessary to arrive at a desired annealing temperature. Such final strip temperature is chosen to be above the recrystallization temperature and below the lower critical or transformation temperature above which the steel strip experiences a dimensional change due to a metallurgical phase change and renders handling of the strip in the furnace in multiple-pass fashion difficult, if not impossible. In order to limit furnace sizes to practical dimensions while affording a reasonable rate of production in the presence of a progressively-diminishing temperature differential between the radiant heating means and the strip as it travels through the furnace, however, it has been the practice to operate the furnace at a temperature above the aforementioned recrystallization temperature as well as above the critical temperature of such strip, hundreds of degrees in excess of the final temperature to which the strip is heated for annealing.

Such practice of employing a furnace operated at a temperature above the recrystallization temperature as well as above the critical temperature to heat the strip to the annealing temperature exclusively by radiation and at relatively high rates of travel imposes certain limitations with respect to the operating facilities and characteristics which must be provided or accommodated in such method.

For example, a looping tower at the entry to the furnace is employed which is capable of storing a considerable length of tinplate strip to enable the following portion of the strip to continue travel through the furnace while its trailing edge at entrance to the looping tower is temporarily halted for welding to the forward edge of a succeeding strip roll. This prevents overheating of the strip to the excessive furnace temperature, at which temperature, in present practice, the strip is relatively weak and prone to separate easily under influence of the tensioning means, and at which temperature, being above both the aforementioned recrystallization and critical temperatures, the strip could experience physical, metallurgical and undesired dimensional changes. The large thermal inertia of the furnace affords no opportunity for short-term furnace temperature reduction to serve this end. In furnace annealing lines operating at two thousand feet per minute for example, and a welding period of thirty seconds, for example, such entry looping tower must be capable of storing a thousand feet of strip, and is necessarily costly, complex, and somewhat difiicult to maintain.

By way of another example, the strip must be maintained in tension in order to enable it to properly track the pass rolls which direct its multiple loop path through the furnace. The amount of tension required is a function of strip speed, and since the practice has been to heat the strip to an annealing temperature exclusively by radiation, the transit speed of the strip through such furnace, hence, speed of the annealing line, therefore has been limited to some degree by the amount of tension which may be applied to the strip while near or at the annealing temperature.

In view of the foregoing remarks, it becomes a prime object of the present invention to provide a method and apparatus for continuous annealing metal strip, which employs a high thermal inertia heating means, such as a radiant furnace, and low thermal inertia heating means, such as induction heating coil means, constructed and arranged to afford opportunity for temporary line shutdown without strip overheating and without need for an entry looping tower, increased operating speed of the annealing line, rapid and precise heat control, as well as to afford other advantages which hereinafter will become apparent from the following description of the invention when taken in connection with the accompanying drawing, in which:

FIGURE 1 is a schematic representation of the novel annealing apparatus for the continuous annealing of metal strip in accord with the present invention; and

FIG. 2 is a curve showing the temperature of the strip in uninterrupted transit through the apparatus of FIG. 1.

Referring to the drawings, in accord with the invention, the strip 1 unwinds from the usual coil holder 2 and is fed by positioning rolls 3 past a welding station 4 through such as a first series of pinch rolls 5 and bridles 6, through a tension control means 7 into a furnace 8 of conventional structure, the interior of which includes radiant heating elements 9 operated at a temperature less than the lower critical temperature of the strip material as well as less than at least the recrystallization tempera ture for the strip at running and temporary shutdown conditions, less than about 1100 F., for example, or at about 1050 F., for example, as in annealing 0.015 inch thick, or less, to cold-reduced low carbon steel tinplate stock being heated during running, and which temperature is one which will afford increased strip strength during running conditions and will not adversely affect the sought-after metallurgical properties of the strip, should the strip be subjected to this temperature for a prolonged period of time, as during strip-joining welding, or temporary shutdown, for example.

Durin travel of the strip through the furnace its speed will be such that it becomes heated radiantly to a subrecrystallization temperature, 900 F. to 950 F. for ex ample, less than the final strip temperature, but of suf'fi cient differential with respect to the exemplified 1050" F. temperature of the radiant heating elements 9 and of the furnace walls to assure a reasonably rapid radiant heat transfer therebetween.

Finally, preferably within the furnace 8, the strip is fed through or past low thermal inertia heating means such as induction heating coil means 11 normally energized to raise the temperature of the strip rapidly from the sub-recrystallization temperature of 950 F., for example, up to its preferred final annealing temperature above the recrystallization temperature of the strip, which, in the case of greater-than-60% cold-reduced steel tinplate material as presently composed, may be within the range of 1200 F. to 1350 F., for example. From the induction coil means 11, the strip may pass through a means providing a soak zone (not shown) and/or a cooling zone (not shown) as final steps associated with the annealing of the strip.

By means of the apparatus of the present invention and thus far described, the line may be stopped temporarily, the induction heating coil means 11 deenergized, and the strip, although stopped within the high thermal inertia heating means, the exemplified furnace 8, may be allowed to rise to the limited furnace temperature without becoming adversely affected by even a prolonged presence in the furnace. This affords opportunity, for example, to weld a new strip roll to the tail end of the first strip without need for reliance on an entry looping tower.

In addition, during normal operation of the line, since the strip is approximately several times stronger in tension at a temperature immediately below its recrystallization temperature than it is immediately above such temperature, in the case of certain metals such as tinplate stock exemplified herein, it is apparent that, in accord with the present invention, more tension may be applied to the strip within the furnace up to final entry into the induction coil means, thereby affording opportunity for increased travel speed of the strip without risk of strip separation within the furnace, which requires a time-consuming rethreading operation. Toward this end, further advantages may be realized by limiting the effect on the strip of tension control means 7 to a region exclusive of the induction heated region by location of such tension control means between the first rotary strip gripping means 5 and 6 at input to the furnace and a second similar means 5 and 6 at entrance to the coil means 11. Thus, the means 7 will not apply tension to the strip 1 at the higher coil exit temperature.

In addition, by inclusion of such as the induction heating coil means 11 as low-thermal-inertia, quick-acting final-stage heating means, the strip annealing line is susceptible to precise and rapid heat control which readily accommodates a relatively high degree of automation. In this behalf, further advantage of the invention can be taken by the inclusion of strip temperature sensing means 12 cooperable with induction coil energization control means 14 to regulate the power from a high frequency power source 15 to the heating coil means 11. By virtue of such arrangement, the final annealing temperature of the strip may be rapidly, automatically, and accurately controlled.

In summary, in accord with the basic concepts of the invention, by employment of the apparatus disclosed herein a continuous strip of metal, such as low carbon steel, is annealed by first heating it in high thermal inertia heating means, such as within a multi-loop high-thermal-inertia furnace to a temperature at least less than the recrystallization temperature of the strip at normal running conditions of the apparatus, including temporary stopping of the strip within such heating means, thereby affording the increased tensile strength of the strip at the sub-recrystallization temperature during running conditions and avoidance of strip overheating during such temporary stopping; followed by heating the radiantlyheated strip rapidly by low thermal inertia heating means, such as induction heating coil means, to bring the strip up to final anneal temperature. To take further advantage of the increased strip strength, the present invention additionally includes a strip tensioning means which is divorced from the strip as heated to final anneal temperature by the induction heating coil means. By virtue of this apparatus wherein a low thermal inertia heating means such as the induction heating coil means, rather than a high thermal inertia heating means such as a radiant furnace, performs the final heating of the strip to above the recrystallization temperature of the strip, the final temperature desired for annealing is freed from consideration of strip separation as is of concern in such as tracking through a vertical multiple loop path with a furnace, and opens the door to shut-down of the line without danger of overheating the strip as well as possibly to experimentation, for example, with annealing temperatures even above the critical or transformation temperature of metal, such as low carbon steel, which experiences dimensionally-influencing phase changes when heated above these temperatures and renders tracking through a long vertical, circuitous path diflicult and practically impossible to maintain. Automatic and rapid control of the final annealing temperature of the strip also is afforded by virtue of the susceptability of the low inertia heating means, induction heating coil means as exemplified, to such control, in contrast to the high thermal lag inherent in a large radiant furnace, for example.

Whereas the invention is related to the recrystallization temperature of the metal strip being annealed, sometimes referred to as the recrystallization threshold temperature, which temperature for a given metal and given preanneal conditions will vary according to the length of time at temperature, the longer the time at temperature, the lower the recrystallization temperature; by suitable further reduction of furnace operating temperature tolerance of the strip to stoppage within the radiant section without recrystallizing effect can be prolonged, if desired, beyond the usual, planned, temporary shut-downs for strip-end welding, for example.

Further, wherein the invention relates to heating steel strip to a maximum temperature less than the Curie-temperature of such strip by induction heating coil means, as in fast annealing cold Worked low carbon steel tinplate strip, advantage can be realized in the increased efficiency and simplicity of such heating mode at sub-Curie-temperature heating relative to above-Curie-temperature heating by such heating mode. Still further, where a maximum strip temperature in the immediate vicinity of such Curie temperature is feasible, heating effect of such a coil in a simple helical form tends to become self-regulating at such temperature.

Although the invention has been described with a degree of particularity, it will be understood that this is by way of illustration, and that the invention embraces modifications and other uses which will be obvious to those versed in the art, within the scope of the appended claims.

I claim as my invention:

1. Apparatus for the continuous annealing of metal strip, comprising a multiple-loop radiant furnace for heating such strip radiantly while in transit therethrough to a temperature less than its recrystallization temperature, induction heating coil means at the exit of such furnace for heating the radiantly-preheated strip up to above said recrystallization temperature, and power regulating control means for said induction heating coil means constructed and arranged to automatically regulate strip temperature during running operation of the apparatus and to render said induction heating coil means ineffective when the travel of strip through the furnace is halted.

2. Apparatus for the continuous annealing of metal strip, comprising a multiple-loop radiant furance for heating such strip radiantly While in transit therethrough to a temperature less than its annealing temperature, induction heating coil means at the exit of such furance for heating the radiantly-preheated strip up to the final annealing temperature, automatically-regulated strip tensioning means for assuring proper tracking of the strip While in transit through the multiple loops in said furance, and means following such multiple loops and preceding said induction heating coil means for divorcing said strip as heated by said induction heating coil means from influence by said tensioning means.

References Cited UNITED STATES PATENTS 1,493,140 5/1924 Zimmerman 148134 1,581,269 4/1926 Kelley 14812 2,434,599 1/1948 Stoltz 2663 2,448,835 9/1948 Schefe 2663 2,504,440 4/ 1950 Miess 219-l0.61 X 2,897,698 8/1959 Van Orrner 2663 X CHARLES N. LOVELL, Primary Examiner. 

